Scientists have been working on a way to store extra energy from off-peak times to use during peak electricity demand. They have been testing a method called compressed air energy storage (CAES) in underground reservoirs. Different types of reservoirs have been studied, and a field test in Illinois showed promising results, even though the injection of air was slower than expected. The US Department of Energy initially sponsored the test, but it was later taken over by the Electric Power Research Institute. The test site in Pittsfield, Illinois, is being used to study how porous sandstone reservoirs behave under CAES conditions.